In optical transmission systems, devices have been put into practical use for monitoring an OSNR (Optical Signal-to-Noise Ratio) so as to detect the state or failure of, for example, an optical transmission device, an optical transceiver, or an optical path. In regard to a next-generation optical network, it is predicted that the wavelength channels and/or degrees of optical signals may be dynamically switched, and hence there will be a higher need for monitoring an optical signal quality.
In implementing an OSNR monitor, a main signal (e.g., optical signal for transmission of data) needs to be unaffected. Meanwhile, a specified monitoring accuracy is requested even for polarization multiplexed optical signal or an optical signal with dispersion (e.g., chromatic dispersion or polarization mode dispersion). Moreover, a simple and low-cost configuration is needed.
The following device is known as an exemplary OSNR monitor with a simple configuration. That is, an optical splitter guides optical signals to first and second paths. An optical power measuring unit measures the signal intensity of the optical signal on the first path. A noise measuring unit processes AC components of selectively passed optical signal on the second path and measures the noise intensity of the processed AC components. An OSNR calculator calculates the OSNR of the optical signal by comparing the measured signal intensity and the measured noise intensity. (See, for example, U.S. Pat. No. 6,433,864)
Note that Japanese National Publication of International Patent Application No. 2003-526232 and Japanese Laid-open Patent Publication No. 2013-12834 describe related arts.
In recent years, polarization multiplexing has been put into practical use as a technique for increasing the capacity of optical signals. Polarization multiplexing transmits data using a pair of polarized waves that are orthogonal to each other.
However, according to the simple configuration described above (e.g., configuration described in U.S. Pat. No. 6,433,864), an OSNR may be unable to be accurately measured depending on the state of polarization of optical signals. For example, according to the simple configurations described above, the accuracy of measurement of an OSNR may be decreased due to a PDL (Polarization Dependent Loss).